Chapter 100. Megaloblastic Anemias (Part 5)

Chia sẻ: Thuoc Thuoc | Ngày: | Loại File: PDF | Số trang:5

Thêm vào BST

Báo xấu

67
lượt xem 4
download

Download Vui lòng tải xuống để xem tài liệu đầy đủ

An underlying maternal folate metabolic abnormality has also been postulated. One abnormality has been identified: reduced activity of the enzyme 5,10-methylene-THF reductase (MTHFR) (Fig. 100-1) caused by a common 677CÆT polymorphism in the MTHFR gene. In one study, the prevalence of this polymorphism was found to be higher in the parents of NTD fetuses and in the fetuses themselves: homozygosity for the TT mutation was found in 13% compared with 5% in control subjects. The polymorphism codes for a thermolabile form of MTHFR. The homozygous state results in a lower mean serum and red cell folate level compared with...

Chủ đề:

Bình luận(0) Đăng nhập để gửi bình luận!

Lưu

Nội dung Text: Chapter 100. Megaloblastic Anemias (Part 5)

Chapter 100. Megaloblastic Anemias (Part 5) An underlying maternal folate metabolic abnormality has also been postulated. One abnormality has been identified: reduced activity of the enzyme 5,10-methylene-THF reductase (MTHFR) (Fig. 100-1) caused by a common 677CÆT polymorphism in the MTHFR gene. In one study, the prevalence of this polymorphism was found to be higher in the parents of NTD fetuses and in the fetuses themselves: homozygosity for the TT mutation was found in 13% compared with 5% in control subjects. The polymorphism codes for a thermolabile form of MTHFR. The homozygous state results in a lower mean serum and red cell folate level compared with control subjects, as well as significantly higher serum homocysteine levels. Tests for mutations in other enzymes possibly
associated with NTDs, e.g., methionine synthase or serine–glycine hydroxymethylase, have been negative. Autoantibodies to folate receptors have, however, been detected in 9 of 12 women who were or had been pregnant with a fetus with a NTD, but in only 2 of 20 control women. Antiserum to folate receptors results in resorption or multiple developmental abnormalities in mouse embryos. It is possible, therefore, that the association of antibodies to maternal folate receptors and NTDs reflects a causal relation. Cardiovascular Disease Children with severe homocystinuria (blood levels ≥100 µmol/L) due to deficiency of one of three enzymes, methionine synthase, MHTFR, or cystathionine synthase (Fig. 100-1), suffer from vascular disease, e.g., ischemic heart disease, cerebrovascular disease, or pulmonary embolus as teenagers or in young adulthood. Lesser degrees of raised serum homocysteine and low levels of serum folate have been found to be associated with cerebrovascular, peripheral vascular, and coronary heart disease and with deep vein thrombosis. Prospective randomized trials of lowering homocysteine levels with supplements of folic acid, vitamin B12, and vitamin B6 against placebo over a 5-year period in patients with vascular disease or diabetes have not, however, shown a reduction of major cardiovascular events, nor have these supplements reduced the risk of recurrent
cardiovascular disease after an acute myocardial infarct. It is possible that these trials were not sufficiently powered to detect a small (e.g., 10%) benefit or that some other underlying factor is responsible for both the vascular damage and the raised homocysteine. Alternatively, the beneficial effects of lowering homocysteine were offset in these trials by the vitamins stimulating endothelial cell proliferation. The results of longer and larger trials are needed to resolve these uncertainties. Malignancy Prophylactic folic acid in pregnancy has been found to reduce the subsequent incidence of acute lymphoblastic leukemia (ALL) in childhood. A significant negative association has also been found with the MTHFR677(C→T) polymorphism and leukemias with mixed lineage leukemia (MLL) translocations, but a positive association with hyperdiploidy in infants with ALL or acute myeloid leukemia or with childhood ALL. A second polymorphism in the MHTFR gene, A1298C, is also strongly associated with hyperdiploid leukemia. There are various positive and negative associations between polymorphisms in folate-dependent enzymes and the incidence of adult ALL. The C677T polymorphism is thought to lead to increased thymidine pools and "better quality" of DNA synthesis by shunting 1-carbon groups towards thymidine and purine synthesis. This may explain its reported association with a lower risk for colorectal cancer. Other
tumors that have been associated with folate polymorphisms or status include follicular lymphoma, breast cancer, and gastric cancer. Neurologic Manifestations Cobalamin deficiency may cause a bilateral peripheral neuropathy or degeneration (demyelination) of the posterior and pyramidal tracts of the spinal cord and, less frequently, optic atrophy or cerebral symptoms. The patient, more frequently male, presents with paresthesias, muscle weakness, or difficulty in walking and sometimes dementia, psychotic disturbances, or visual impairment. Long-term nutritional cobalamin deficiency in infancy leads to poor brain development and impaired intellectual development. Folate deficiency has been suggested to cause organic nervous disease but this is uncertain, although methotrexate injected into the cerebrospinal fluid may cause brain or spinal cord damage. An important clinical problem is the nonanemic patient with neurologic or psychiatric abnormalities and a low or borderline serum cobalamin level. In such patients, it is necessary to try to establish whether or not there is significant cobalamin deficiency, e.g., by careful examination of the blood film, cobalamin absorption studies, tests for antibodies to IF or parietal cells, and serum methylmalonic acid (MMA) measurement if available. A trial of cobalamin
therapy for at least 3 months will also usually be needed to determine whether the symptoms improve. The biochemical basis for cobalamin neuropathy remains obscure. Its occurrence in the absence of methylmalonic aciduria in TC II deficiency suggests that the neuropathy is related to the defect in homocysteine-methionine conversion. Accumulation of S-adenosylhomocysteine in the brain, resulting in inhibition of transmethylation reactions, has been suggested.